There is a need for high performance coatings that can be applied with low levels of polluting solvents and monomers. Aqueous-based thermoplastic coatings can be applied with low levels of polluting solvents, but they do not have the heat and chemical resistance required for many applications, especially where parts must be stacked soon after coating and where the substrate cannot be heated to high temperatures. Chemically-cured coatings that give good heat and chemical resistance have problems with pot life and cure speed.
Multifunctional materials may be blended with aqueous latex polymers to give a UV-curable composition. Problems are encountered with these two component UV-curable compositions:
First, the multifunctional materials, such as for example monomers and oligomers, may present safety, health and environmental problems.
Second, problems are encountered with mixing the multifunctional materials with the latex polymers, including incompatibility of the two components and the burden and inefficiency of additional steps. The multifunctional material may have to be pre-emulsified before it can be added to the latex polymer. Improper pre-emulsification leads to defects in the final UV-cured coating.
Incorporating the multifunctionality into the polymer provides a solution to these problems. A number of patents and publications have disclosed aqueous UV-curable compositions formed from functional latex polymer emulsions:
U.S. Pat. No. 4,107,013 teaches a UV-curable aqueous latex paint containing a latex and 5-35% of an emulsified low molecular weight crosslinking agent wherein the latex has a shell of copolymerized difunctional monomers containing unreacted allyl surface groups.
U.S. Pat. No. 4,244,850 teaches an air-curing latex coating composition containing unsaturated resin particles, drier salt emulsion and water-immiscible organic solvent. The unsaturated resin is formed from 1-20% by weight of a monoethylenically unsaturated monomer having a carboxyl or 1,2-epoxy functionality, a portion of which has been reacted with 1,2-epoxy or carboxyl functionality, respectively, to provide unsaturation sites on the resin particles. The resin is not neutralized before functionalization.
European Patent Application EP 330,246 teaches a curable dispersion formed by first polymerizing an organic phosphate or phosphonate compound or mixtures thereof with a (meth)acrylate derivative or an other unsaturated compound or a styrene derivative and then adding an ethylenically unsaturated, epoxy-containing monomer.
U.S. Pat. No. 4,925,893 teaches auto-oxidative and radiation curable vinylidene chloride/vinyl chloride/2-ethylhexyl acrylate latices having residual unsaturation via the addition of a gel fraction of at least 5 weight % formed from a multifunctional monomer early in the polymerization and the addition of a low reactivity multifunctional compound, such as diallyl phthalate, late in the polymerization.
European Patent Application EP 442,653 teaches the preparation of polymers containing functionality. An amine functional latex is formed by reacting a carboxyl functional latex with aziridines. The amine functional latex is then reacted with a material having both an enolic carbonyl group and another functional group, for example 2-(acetoacetoxy)ethyl methacrylate to give a methacrylate functional polymer.
Loutz et al. [Organic Coatings, Number 8, pages 197-209 (1986)] teach the preparation of emulsion according to a core-shell introduction of the feed pre-emulsion in a 7/3 ratio. The difunctional monomer is contained in the shell pre-emulsion.
The foregoing exemplifies numerous prior attempts to make crosslinkable, UV-curable coatings from polymer emulsions having functionality. However, none of these references teaches or suggests a multistage latex polymer having .alpha., .beta.-unsaturated carbonyl functionality in the outer stage or its preparation.